This invention relates to a process for improving oil recovery from stratified reservoirs.
Brine-tolerant surfactants may be injected at a salinity such that viscous emulsions or microemulsions are promoted by mixing of surfactant solution with crude oil and formation brine. (Note U.S. Pat. Nos. 4,160,480; 4,161,218; 4,161,982; 4,161,983; 4,165,785; 4,184,549; 4,194,564; and 4,307,782, the entire disclosures of which are expressly incorporated herein by reference.) For example, U.S. Pat. No. 4,307,782 teaches injection of surfactant solution at a salinity 5 to 20 percent below that necessary for partitioning greater than 50 percent of surfactant from brine solutions into an oil phase, or into an oil-water interface. In the process, the partitioning occurs when the lower salinity surfactant solution mixes with the higher salinity formation brine. It has been observed that these surfactants and emulsions formed from them are very difficult to propagate at salinities conducive to forming the microemulsion phases. The method proposed in U.S. Pat. No. 4,307,782 depends upon injection of lower salinity brine to release phase trapped surfactant and emulsions--advancing them by a series of steps in which emulsions are formed in the front mixing zone, trapped, released by lower salinity drive, advanced, reformed by front mixing, etc.
When brine tolerant surfactants, e.g., alkyl- and alkylarylpolyethoxysulfonates or sulfates, sometimes in combination with non-ionic surfactants or petroleum sulfonates, are injected into stratified reservoirs according to the teachings of the above-mentioned patents, emulsion phases are immediately formed which increase flow resistance in strata penetrated by them. Because high permeability strata accept relatively more of injected fluids, and because such surfactants and emulsion phases propagate more readily in high permeability zones, flow resistances are increased relatively more in the high permeability zones.
An objective of this invention is to provide salinity control measures to (1) improve the rate and depth of penetration by surfactant/polymer solutions into high permeability zones, and (2) to retain viscous emulsions within the high permeability zones while subsequently injected waters are diverted into the lower permeability zones.